Measurement of film thickness during growth is required in a number of arts and especially in the growth of thin film semiconductors on substrates. Such films may comprise amorphous or polycrystalline or epitaxial films. Such films may be grown or deposited in a variety of reactors. One such reactor is a molecular beam epitaxy (MBE) reactor.
Recently, the examination of images formed by in situ reflection high-energy electron diffraction (RHEED) intensity data has become one of the more useful tools for the analysis of growth by MBE. Static RHEED images, obtained when growth is interrupted, yield detailed information on surface reconstruction. Analysis of dynamic RHEED images ("RHEED oscillations"), plotted as a function of intensity versus time, and obtained as growth is taking place, can be employed to determine epitaxial growth rates and, therefore, alloy compositions. (See U.S. Pat. No. 4,855,013 issued Aug. 8, 1989.) This method is applicable because epitaxial growth causes a variation in surface roughness on the atomic scale that, under favorable conditions, produces well-resolved oscillations in the RHEED intensity. The period of these oscillations is the time required for the growth of one complete monolayer. Frequently, however, the conditions used for epitaxial growth yield oscillations that are not sufficiently obvious to permit the period to be obtained directly from plots of intensity versus time. Another major disadvantage with current known RHEED oscillation analysis methods is that substrate rotation, which is generally employed in order to improve uniformity of the growing films and interfaces, must be stopped in order to obtain a fixed diffraction pattern from which oscillation data can be obtained.
A need exists, therefore, for a method and apparatus which permits analysis of RHEED or other oscillations under unresolved growth conditions and which permits the acquisition of RHEED oscillation data while the substrate continues to rotate.